Nucleic-Acid Transfection
Nucleic acids can be delivered to cells both in vitro and in vivo by pre-complexing the nucleic acids with charged lipids, lipidoids, peptides, polymers or mixtures thereof. Such transfection reagents are commercially available, and are widely used for delivering nucleic acids to cells in culture. Cells exposed to transfection reagent-nucleic acid complexes may internalize these complexes by endocytosis or other means. Once inside a cell, the nucleic acid can carry out its intended biological function. In the case of protein-encoding RNA, for example, the RNA can be translated into protein by the ribosomes of the cell.
Serum-Free Cell Culture
Animal sera such as fetal bovine serum (FBS) are commonly used as a supplement in cell-culture media to promote the growth of many types of cells. However, the undefined nature of serum makes cells that are contacted with this component undesirable for both research and therapeutic applications. As a result, serum-free cell-culture media have been developed to eliminate the batch-to-batch variability and the risk of contamination with toxic and/or pathogenic substances that are associated with serum.
The most abundant protein in serum is serum albumin. Serum albumin binds to a wide variety of molecules both in vitro and in vivo, including hormones, fatty acids, calcium and metal ions, and small-molecule drugs, and can transport these molecules to cells, both in vitro and in vivo. Serum albumin (most often either bovine serum albumin (BSA) or human serum albumin (HSA)) is a common ingredient in serum-free cell-culture media, where it is typically used at a concentration of 1-10 g/L. Serum albumin is traditionally prepared from blood plasma by ethanol fractionation (the “Cohn” process). The fraction containing serum albumin (“Cohn Fraction V” or simply “Fraction V”) is isolated, and is typically used without further treatment. Thus, standard preparations of serum albumin comprise a protein part (the serum albumin polypeptide) and an associated-molecule part (including salts, fatty acids, etc. that are bound to the serum albumin polypeptide). The composition of the associated-molecule component of serum albumin is, in general, complex and unknown.
Serum albumin can be treated for use in certain specialized applications (See Barker A method for the deionization of bovine serum albumin. Tissue Culture Association. 1975; Droge et al. Biochem Pharmacol. 1982; 31:3775-9; Ng et al. Nat Protoc. 2008; 3:768-76; US Patent Appl. Pub. No. US 2010/0168000, the contents of which are hereby incorporated by reference). These treatment processes are most commonly used to remove globulins and contaminating viruses from solutions of serum albumin, and often include stabilization of the serum albumin polypeptide by addition of the short-chain fatty acid, octanoic acid, followed by heat-inactivation/precipitation of the contaminants. For highly specialized stem-cell-culture applications, using an ion-exchange resin to remove excess salt from solutions of BSA has been shown to increase cell viability (See Ng et al. Nat Protoc. 2008; 3:768-76; US Patent Appl. Pub. No. US 2010/0168000, the contents of which are hereby incorporated by reference). However, recombinant serum albumin does not benefit from such treatment, even in the same sensitive stem-cell-culture applications (See Ng et al. Nat Protoc. 2008; 3:768-76; US Patent Appl. Pub. No. US 2010/0168000, the contents of which are hereby incorporated by reference), demonstrating that the effect of deionization in these applications is to remove excess salt from the albumin solution, and not to alter the associated-molecule component of the albumin. In addition, the effect of such treatment on other cell types such as human fibroblasts, and the effect of such treatment on transfection efficiency and transfection-associated toxicity have not been previously explored. Furthermore, albumin-associated lipids have been shown to be critical for human pluripotent stem-cell culture, and removing these from albumin has been shown to result in spontaneous differentiation of human pluripotent stem cells, even when lipids are added separately to the cell-culture medium (See Garcia-Gonzalo et al. PLoS One. 2008; 3:e1384, the contents of which are hereby incorporated by reference). Thus, a cell-culture medium containing albumin with an unmodified associated-molecule component is thought to be critical for the culture of human pluripotent stem cells. Importantly, the relationship between the associated-molecule component of lipid carriers such as albumin and transfection efficiency and transfection-associated toxicity has not been previously explored.
Cell Reprogramming
Cells can be reprogrammed by exposing them to specific extracellular cues and/or by ectopic expression of specific proteins, microRNAs, etc. While several reprogramming methods have been previously described, most that rely on ectopic expression require the introduction of exogenous DNA, which can carry mutation risks. DNA-free reprogramming methods based on direct delivery of reprogramming proteins have been reported, however these methods are too inefficient and unreliable for commercial use. In addition, RNA-based reprogramming methods have been described, however, existing RNA-based reprogramming methods are slow, unreliable, and inefficient when performed on adult cells, require many transfections (resulting in significant expense and opportunity for error), can reprogram only a limited number of cell types, can reprogram cells to only a limited number of cell types, require the use of immunosuppressants, and require the use of multiple human-derived components, including blood-derived HSA and human fibroblast feeders. The many drawbacks of previously disclosed cell-reprogramming methods make them undesirable for both research and therapeutic use.
Gene Editing
Several naturally occurring proteins contain DNA-binding domains that can recognize specific DNA sequences, for example, zinc fingers (ZFs) and transcription activator-like effectors (TALEs). Fusion proteins containing one or more DNA-binding domains and the catalytic domain of a nuclease can be used to create a double-strand break in a desired region of DNA in a cell. When combined with a DNA template containing one or more regions of homology to the DNA of the cell, gene-editing proteins can be used to insert a DNA sequence or to otherwise alter the sequence of the DNA of the cell in a controlled manner. However, most current methods for gene editing cells use DNA-based vectors to express gene-editing proteins. As a result, these gene-editing methods are inefficient, and carry a risk of uncontrolled mutagenesis, making them undesirable for both research and therapeutic use. Methods for DNA-free gene editing of somatic cells have not been previously explored, nor have methods for simultaneous or sequential gene editing and reprogramming of somatic cells. Finally, the use of gene editing in an anti-bacterial, anti-viral, or anti-cancer treatment has not been previously explored.
Model Organisms
Knockout rats have been generated by embryo microinjection of nucleic acids encoding zinc-finger nucleases and TALE-nucleases (TALENs). Gene editing to introduce sequence-specific mutations (a.k.a. “knockins”) has also been reported in mice and rats by injecting nucleic acids encoding zinc-finger nucleases into embryos. Genetically-modified rats have been generated using embryonic stem cells, and germline-competent rat pluripotent stem cells have been generated by somatic-cell reprogramming. However, the use of gene-edited reprogrammed cells to generate genetically modified organisms, including mice and rats has not been previously explored.
There is a need in the field for improved methods and products for transfecting cells.